Engineering topics

Graves, J. A.

Publications and source records attributed to Graves, J. A..

4 records

NASA NTRS1987

Containerless processing of undercooled melts

Containerless drop tube processing allows for significant levels of liquid undercooling through control of parameters such as sample size, surface coating and cooling rate. A laboratory scale (3 m) drop tube has been developed which allows the undercooling and solidification behavior of powder samples to be evaluated under low gravity free-fall conditions. The level of undercooling obtained in an InSb-Sb eutectic alloy has been evaluated by comparing the eutectic spacing in drop tube samples with a spacing/undercooling relationship established using thermal analysis techniques. Undercoolings of 0.17 and 0.23 T(e) were produced by processing under vacuum and He gas conditions respectively. Alternatively, the formation of an amorphous phase in a Ni-Nb eutectic alloy indicates that undercooling levels of approximately 500 C were obtained by drop tube processing. The influence of droplet size and gas environment on undercooling behavior in the Ni-Nb eutectic was evaluated through their effect on the amorphous/crystalline phase ratio. To supplement the structural analysis, heat flow modeling has been developed to describe the undercooling history during drop tube processing, and the model has been tested experimentally.

Shong, D. S.↗

NASA NTRS1987

Rapid solidification of highly undercooled liquids

Melt undercooling is considered as a fundamental processing parameter for powders and for melt-spun and surface-melted samples. With reference to experimental results for Mn, Al, Al-Be, Al-Fe, and InSb-Sb powders, it is shown that the undercooling behavior in droplets is influenced by powder size, coating, melt superheat, and cooling rate. Droplet sample microstructures can be compared directly with those of rapidly quenched powders to establish the application of laboratory-scale studies to large-scale processing.

Perepezko, J. H.↗

NASA NTRS1986

Undercooling and crystallization behaviour of antimony droplets

The droplet emulsion technique is presently used to examine the undercooling and crystallization behavior of pure antimony. Control of droplet size and applied cooling rate allowed maximum undercooling to be extended from 0.08 to 0.23 T(m). A droplet coating was produced by means of emulsification which appears to furnish a favorable crystallographic matching for effective nucleation catalysis of a metastable simple cubic structure. Thermal analysis shows the melting temperature of the single cubic phase to be about 625 C.

Graves, J. A.↗

NASA NTRS1985

Undercooling and solidification behavior in the InSb-Sb system

Use of the droplet emulsion technique has been successful in studying the undercooling and crystallization behavior of Sb, InSb, and an InSb-Sb eutectic alloy. Both droplet size and imposed cooling rate were influential in controlling the extent of liquid undercooling. The droplet surface coating was of significant importance in determining the resultant solidification product structure through its effect on nucleation kinetics. The maximum undercooling for pure Sb was extended from 0.08 to 0.23 T sub m. While simple crushing techniques provided a dramatic increase in droplet undercooling over the bulk material, emulsification treatments both enhanced this undercooling and allowed successful formation of a metastable simple cubic Sb phase. This phase was stable to temperatures approaching the melting point. The simple cubic phase was detected in droplet samples processed using DTA, air and water quenching, and drop tube processing under a helium gas atmosphere. A deviation in the InSb parent ingot composition limited interpretation of the line compound results, however, emulsification techniques extended the undercooling of this material to 0.17 T sub L and provided a stable, protective surface coating for the droplets. Emulsification of the eutectic alloy was effective at producing various levels of undercooling from 0.1 to 0.2 T sub E. Microstructural examination revealed a normal-type eutectic structure in the undercooled droplets indicating that solidification occurred within the coupled zone and that this zone is somewhat symmetric about the eutectic composition.

Graves, J. A.↗